LEDs, particularly for the LED lighting industry, are conventionally driven by pulse width modulation (PWM). In PWM, the LED is modulated between an on state and an off state. When in the on state, typically the LED is supplied with a constant current. When in the off state, there is no current is supplied to the LED. The output flux, that is to say the amount of light output by the LED is determined by the time-integral of the current. So by varying the pulse width, while keeping the current in the on state constant, the optical output of the LED can be varied without changing the instantaneous current through the LED.
This is important because the wavelength of the LED can have a strong current dependency. The wavelength can decrease by up to 30 nm/A. Maintaining a constant wavelength of the optical output from the LED can be useful for a single colour LED; however, it is of particular importance for multicoloured LED arrays. Typically in such multicoloured arrays, the outputs of three sets of LEDs having different colours are combined. The apparent colour of the combined array is then dependent on both the ratio of the intensities of the three sets of the LEDs, and on their absolute wavelengths. When the three sets of LEDs are combined to produce white light, it is particularly important to be able to control or maintain the wavelengths of the component LEDs, in order to have accurate control over the “combined colour temperature” (CCT) of the output.
Although PWM has heretofore been the preferred control method particularly for multicolour arrays of LEDs, it still suffers from the disadvantage that both the flux output and the colour of the individual LEDs is still temperature dependent; without compensation, a visible effect on the output can be observed for a temperature difference of merely 20° C.
Using the LED itself to determine the temperature of the LED has been disclosed in international patent application, publication WO-A-2007/090283. This is used to estimate the colour of the LED, whereas the duty cycle of the control is adjusted to control the output flux of the LED.